For purposes of introducing basic concepts, consider the illustrative microgrids of FIGS. 1 and 1A. In FIG. 1, a microgrid comprises a conventional small diesel generator located at network node 1 and connected to a photo-voltaic (PV) panel (also sometimes referred to herein as a “solar panel”) located at network node 2. The same microgrid as that shown in FIG. 1 with a very large generator at node 1 may thus represent, for example, a utility grid connected via node 2 to a small PV panel with its local load. Opening of a switch S represents a sudden fault which results in the PV panel becoming disconnected from the main utility.
FIG. 1A illustrates a stand-alone microgrid having mixed generation sources here illustrated as diesel generation (for example) and solar power. The open switch S indicates a fault (e.g. a sudden and unexpected fault) leading to “islanded” operation of the PV panel. In this scenario, the PV panel must supply its own load and the rest of the load in this example being supplied by a conventional diesel generator (designated as “induction machine” in FIG. 1A). Loads themselves could be diverse and range from constant impedance loads, through common induction motors and variable speed drive motors having their own internal control. Thus, such faults may result in a variation of current flowing out of a PV panel.
It is known that power produced by a solar panel may vary due to a variety of factors including, but not limited to environmental factors (e.g. clouds, rain, etc. . . . ). A typical solar irradiance power output from an illustrative solar panel (which may be the same as or similar to the solar panels described in FIGS. 1, 1A) is shown in FIG. 2. As the current out of a PV panel varies, a PV controller and diesel generator controller must respond to balance the power produced at the respective sources (i.e. the PV panel and induction machine) and the load demand at nodes 1 and 2. A DC/AC PV inverter controller is shown in FIG. 3.
As is known, it is problematic to operate microgrid systems such as those shown in FIGS. 1, 1A both during normal operation and during sudden opening and closing of switch S (i.e. during sudden and unexpected disconnection and reconnection of the PV panel to a utility grid, for example).
This is fundamentally a very difficult control design problem because it concerns operations of highly dynamical nonlinear network systems where topology may change as a of result of switching. This means that the system could have multiple equilibria, some being stable and within acceptable engineering ranges, and others completely non-feasible.